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这是我到目前为止的代码:
template<typename Derived,size_t n = 1> class n_ton_base // Singletons are the default { static Derived instances[n + (n == 0)]; // Zerotons are supported,too protected: // Prevent n_ton_base to be used outside of inheritance hierarchies n_ton_base() {} // Prevent n_ton_base (and Derived classes) from being copied n_ton_base(const n_ton_base&) = delete; public: // Get first element by default,useful for Singletons template<size_t i = 0> static Derived& get_instance() { static_assert(i < n,"Time to increase n it seems!"); return instances[i]; } };
以下是人们如何使用它:
class SingletonExample : public n_ton_base<SingletonExample> { public: void method() { std::cout << "Singletons are overused.\n"; } }; class DoubletonExample : public n_ton_base<DoubletonExample,2> { public: void method() { std::cout << "Doubleton " << this << " says Hello.\n"; } }; int main() { SingletonExample::get_instance().method(); DoubletonExample::get_instance().method(); DoubletonExample::get_instance<0>().method(); DoubletonExample::get_instance<1>().method(); }
不幸的是,代码还没有编译:
/tmp/ccsFtlis.o: In function `SingletonExample& n_ton_base<SingletonExample,1ul>::get_instance<0ul>()': nton.cpp:(.text._ZN10n_ton_baseI16SingletonExampleLm1EE12get_instanceILm0EEERS0_v[SingletonExample& n_ton_base<SingletonExample,1ul>::get_instance<0ul>()]+0x5): undefined reference to `n_ton_base<SingletonExample,1ul>::instances' /tmp/ccsFtlis.o: In function `DoubletonExample& n_ton_base<DoubletonExample,2ul>::get_instance<0ul>()': nton.cpp:(.text._ZN10n_ton_baseI16DoubletonExampleLm2EE12get_instanceILm0EEERS0_v[DoubletonExample& n_ton_base<DoubletonExample,2ul>::get_instance<0ul>()]+0x5): undefined reference to `n_ton_base<DoubletonExample,2ul>::instances' /tmp/ccsFtlis.o: In function `DoubletonExample& n_ton_base<DoubletonExample,2ul>::get_instance<1ul>()': nton.cpp:(.text._ZN10n_ton_baseI16DoubletonExampleLm2EE12get_instanceILm1EEERS0_v[DoubletonExample& n_ton_base<DoubletonExample,2ul>::get_instance<1ul>()]+0x5): undefined reference to `n_ton_base<DoubletonExample,2ul>::instances' collect2: ld gab 1 als Ende-Status zurück
我做错了什么?
template<typename Derived,size_t n = 1> class n_ton_base // Singletons are the default { protected: // Prevent n_ton_base to be used outside of inheritance hierarchies n_ton_base() {} // Prevent n_ton_base (and Derived classes) from being copied n_ton_base(const n_ton_base&) = delete; public: // Get first element by default,"Time to increase n it seems!"); static Derived instance; return instance; } };
请注意,每个实例化的成员函数都有自己的本地静态,因此不需要任何数组.
这也实现了thread-safe lazy initialization,而我不必对此做任何事情.太好了!
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